The present invention relates to a pattern inspection apparatus and, more particularly, to a reference image forming method of forming a reference image for a die-to-database inspection from design data, and a pattern inspection apparatus.
In general, a fine pattern such as a semiconductor integrated circuit formed on a photomask, a reticle, a wafer, a liquid crystal member, or the like must be inspected by using a pattern inspection apparatus to check whether the pattern is accurately drawn in accordance with the size and shape of an ideal pattern formed from design data.
First of all, a pattern inspection apparatus of this type converts design data described with the position coordinates of a rectangle or trapezoid and line segment lengths into binary bit data consisting of "0"s and "1"s corresponding to a target pattern (to be inspected).
The apparatus then scans the target pattern with a laser beam and forms a predetermined optical point divergence function from the pattern information obtained by focusing the transmitted light on a light-receiving element or from the edge profile of the pattern image obtained from an image sensing system. The apparatus converts the design data into multilevel gradation data (multilevel data) by using this function and performing convolution processing with the binary bit data, thereby obtaining a reference image.
The apparatus loads the reference image obtained from the design data in synchronism with the real image obtained by scanning performed by the optical system or input from the image sensing system, and detects incoincident points between the images at corresponding pixel positions, thereby detecting defects on the real pattern (die-to-database inspection).
Note that patterns in a real image have dimensional errors with respect to design ideal values, which cause rounding of corners (corner portions), thickening/thinning of lines, and the like, owing to optical conditions and the influences of a manufacturing process, and pseudo defects that are not determined as defects tend to occur due to pseudo errors with respect to the reference image obtained from the design data.
Under the circumstances, therefore, a reference image may be corrected in advance by performing proper edge position detection or feature extraction such as corner recognition in accordance with the feature amount in each inspection area.
In a conventional reference image forming method, when the pattern line width of a reference image is to be corrected, proper edge detection processing is performed for a real image to correct the pattern correction width of the design data in units of binary data and bits (pixels).
In this case, a changed bit pattern is formed from the design bit pattern, and the bit pattern is EX-ORed with the bit data of the target pattern obtained by binarizing the real image, thereby re-sizing each pattern of the reference image.
In addition, a bit-by-bit basis correction template is prepared, which indicates an edge or corner having a predetermined pattern edge angle (e.g., 0.degree., 45.degree., 90.degree., or 135.degree.) with respect to the edge or corner extracted from the design data by edge position detection, corner recognition, or the like.
Subsequently, the correction template that is most similar to the real image corresponding to the edge or corner is selected to correct the original design data, and the corrected design data is converted into multilevel data again to form a reference image. The gradation difference between the obtained reference image and the real image is checked with the threshold of a proper defect algorithm to check whether the target portion is a defect or not. In this manner, defect detection is performed (see, for example, Japanese Patent Laid-Open No. 4-350776).
In such a conventional reference image forming method and pattern inspection apparatus, original design data is corrected by using a correction template similar to the edge or corner of the reference image formed from the design data, thereby forming a reference image again. In this manner, a reference image similar to the real image data of the normal pattern obtained by optical scanning is formed. For this reason, the inspection throughput decreases owing to the correction processing for the design data, and pseudo defects are produced due to the gradation difference from the correction template.
More specifically, in performing pattern correction for design data, a template similar to the shape of a real image is selected from a plurality of templates of pattern shapes represented by bit strings, and a multilevel reference image for comparison is formed by performing convolution processing for a corrected template image and light intensities calculated from the point divergence function obtained by optically scanning the bit strings obtained by logical operation using the selected template. With this processing, with an increase in the number of pixels, the required calculation amount greatly increases, resulting in a decrease in inspection throughput.
In addition, since the enlargement and reduction widths in pattern correction are determined by arithmetic processing for the bit string based on a template within each pixel, the gradation values of the corrected pattern are limited to the gradation distribution of the template.
Furthermore, since gradation correction is executed regardless of the shapes of adjacent patterns, if a pattern edge is not located at a pixel boundary or a corner portion has a rounded portion, the gradation difference between the reference image and the real image obtained by scanning increases. As a result, pseudo defects tend to occur.
For this reason, at the edge portion of a pattern arranged at an arbitrary angle such that the design pattern has an oblique edge, even if gradation correction is performed in units of pixels by using a template, the gradation differences cannot be interpolated.
Since a defect at a pixel boundary or a pattern defect, on an edge, which exhibits poor contrast with respect to the adjacent portions and is smaller than the inspection resolution, in particular, tends to be similar in gradation to the adjacent portions, the defect detection sensitivity based on comparison processing with a reference image deteriorates.